1. Field of the Invention
The present invention relates to a method and apparatus for mold clamping in an injection molding machine and the like such as plastic injection molding machine or die casting machine, specifically to that for mold clamping to combine a movable die plate with tie bars mechanically during mold close operation of the movable die plate so as to shorten a molding cycle.
2. Description of Related Art
There are disclosed in Japanese laid open patent numbers 10-296809 and 10-296810, methods for mold clamping to combine the movable die plate with the tie bars mechanically during the mold close operation of the movable die plate so as to shorten the molding cycle.
In the former (10-296809), as shown in FIGS. 4 and 5 (not shown here) of it, there is slidably arranged a movable die plate on tie bars which one ends are fastened to a fixed die plate. The movable die plate is provided with a clamping cylinder near a slide portion with the tie bars, and a piston member of the cylinder protrudes from the movable die plate.
In the movable die plate, there is provided with an open and close mechanism mounting nuts formed with multi-split or divided portions called as half nuts, which engage with a screw portion formed at circumference of the tie bars. The nuts engage with the screw portion by driving the open and close mechanism when the movable die plate reaches a position just before a mold close position.
In the former it is disclosed that engagement of the screw portion with the nuts are possible by arranging plural pairs of half nuts even under movement of the movable die plate.
In the latter (10-296810), as shown in FIG. 1 (not shown here) of it, there is arranged a movable die plate providing with a ram cylinder for mold clamping therein and a pair of half nuts on both sides of the movable die plate through which tie bars passe, one end of the tie bars being fastened to a fixed die plate. The one half nuts near the fixed die plate can move in the direction of the tie bars axes independently. At initial setting an appropriate position is defined for engaging with the tie bars. At each molding cycle operation, the other half nuts engages with the tie bars by driving it after the movable die plate contacts the one half nuts at its end surface during the mold close operation. In the case, the distance between both the half nuts is formed to keep at the value of integral multiple to the screw pitch of the tie bars. In the cases mentioned above, the tie bars are fastened to the fixed die plate.
Another arrangement for moving tie bars themselves during the mold close operation is proposed in Japanese laid open patent number 10-296739 wherein an engaging mechanism operates while both a movable die plate and the tie bars move. Such an arrangement for moving the tie bars themselves are explained, referring to FIGS. 6 to 9 attached.
In FIGS. 6 to 9 a reference numeral 10 represents a fixed die plate. The fixed die plate 10 provides with a fixed mold 12. A movable die plate 18 providing with a movable mold 16 is disposed against the fixed die plate 10 and is capable of advancing and retracting thereto.
A nut member 24 is mounted on the under portion of the movable die plate 18. The nut member 24 is screwed with a screw member 22 coupled through a coupling member 22a to a servomotor 20 for the mold open and close operation, which is fastened to the fixed die plate 10 as a driving means.
Rotating of the screw member 22 causes the movable die plate 18 to advance or retract to the fixed die plate 10, thereby executing the mold open and close operation of the movable mold 16 to the fixed mold 12.
Furthermore, half nuts 28 for mold clamping as one of an engaging means are mounted on the left side of the movable die plate 18 through which tie bars 14 are disposed. The tie bars 14 have a screw or groove portion 14a at its one end portion as the other engaging means, to which the half nuts advance and engage by means of a cylinder 26.
Also, die clamping cylinders 30 are mounted on the fixed die plate 10 at its coupling portion with the tie bars 14. A piston 14b formed at the other end portion of the tie bars 14 are slidably inserted in the cylinders 30.
Accordingly, the tie bars 14 are capable of advancing or retracting in the direction of its axis.
Furthermore, as shown in FIG. 8, there is mounted a servomotor 32 on the out side of the cylinders 30. The servomotor 32 is coupled through a coupling member 31a to a screw member 31 screwed into a rod portion 14c of the tie bars 14 at the piston 14b. Accordingly, the tie bars 14 are moved in the direction of the axis by rotating the servomotors 32. Besides, in FIG. 6, a numeral 34 represents a nozzle portion of the injection molding machine.
In such a complex mold clamping apparatus as illustrated in FIGS. 6 to 9, when works are executed for mounting mold 12 and 16 on the fixed die plate 10 and the movable die plate 18, and for adjusting a distance between them, the engagement between the tie bars 14 and the movable die plate 18 is released by the half nuts 28 to allow the mold close operation starting from a most retracted or opened position.
Then, the servomotor 20 executes the mold close operation. In the case, the half nuts 28 is closed at the most advanced or closed position by operating the cylinders 26.
Usually, it is difficult to appropriately engage the half nuts 28 with the screw portion 14a, because of a phase shift between a screw thread and a screw core of thread in both the half nuts 28 and the screw portion 14a. 
To avoid the effect of such a phase shift, the servomotors 32 for moving the tie bars 14 drive it with micro motion, and the servomotors 32 stops to move it at a position that the half nuts 28 and the screw portion 14a are capable of engaging. Then, as shown in FIG. 7, the half nuts 28 comes into appropriate engagement with the screw portion 14a. A position detector (not shown) detects the position of the tie bars 14 corresponding to the appropriate engagement, and the detected value is stored in a memory of a controller (not shown).
Accordingly, in the molding operation after setting the appropriate position, the servomotors 32 controls movement of the tie bars 14 so that the movable die plate 18 always locates at the most advanced position. Thus, the adjusting work finishes by determining the distance between the fixed die plate 10 and the movable die plate 18 when the movable mold 16 contacts with the fixed mold die 12 in the die close operation.
According to the complex mold clamping apparatus as mentioned above, the movable die plate 18 shown in FIG. 6 advances by driving the servomotor 20 after adjusting the engaging position of the half nuts 28 for mold clamping. When the movable die plate 18 reaches a position “B” in FIG. 9 near the most advanced position, the tie bars 14 starts to move in the direction of right in FIG. 9 by driving the servomotors 32, then the servomotors 20 and 32 are controlled so as to synchronize in the relative moving speed between the movable die plate 18 and the tie bars 14, namely so as to become zero in the relative moving speed.
When the relative moving speed between the movable die plate 18 and the tie bars 14 synchronizes at the position “A” in FIG. 9 and then it becomes zero at a specific position, the half nuts 28 operates to close, thereby engaging with the screw portion 14a of the tie bars 14 as shown in FIG. 9.
Also, according to the complex mold clamping apparatus, as mentioned above, molding operation including the mold close operation illustrated in FIG. 9 starts after adjusting the position of the half nuts 28 to engage with the screw portion 14a. 
In the molding operation, first, the movable die plate 18 shown in FIG. 6 advances by driving the servomotor 20. When the movable die plate 18 reaches a position “B” in FIG. 9 near the most advanced mold close position, the tie bars 14, which already moved in the left from the position stored in the memory, starts to move in the direction of right in FIG. 9 by driving the servomotors 32, then the servomotors 20 and 32 are controlled so as to synchronize in the relative moving speed between the movable die plate 18 and the tie bars 14, namely so as to become zero in the relative speed, the relative moving speed being processed in the controller.
Then, an engagement between the half nuts 28 and the screw portion 14a is detected at the position “A” when the relative moving speed in the direction of right between the movable die plate 18 and the tie bars 14 is synchronized. In case that the half nuts 28 is at a position appropriate for the engagement the half nuts 28 is operated to close and engage with the screw portion 14a, holding the relative moving speed to be zero by the controller.
When the engagement is completed and the movable die plate 18 reaches the most advanced or close position, the position detector detects a position of the tie bars 14. The detected value is compared with that stored in the memory as the position of the tie bars 14 when adjusting the distance between the movable die plate 18 and the fixed die plate 10. In case of coincidence between the values, the mold clamping and boosting operation starts.
On the other hand, in case of inappropriate position for engagement, the speed of either one of servomotors 20 or 32 is adjusted. In FIG. 9, the servomotors 32 for slidably moving the tie bars 14 are controlled variably in speed as shown in dotted lines. When the tie bars 14 reach an appropriate position “C” in FIG. 9 for engagement, the speed of the tie bars 14 returns to the relative moving speed in synchronization as before. Then, as stated, the half nuts 28 are operated to close and engage with the screw portion 14a, holding the relative moving speed to be zero by the controller.
In this case, without appropriately positioning for engagement, it is also possible to smoothly close the half nuts 28 in such a way that the half nut 28 is provided with wider spiral or square grooves, which engage with the screw portion 14a of the tie bars 14, so as to form the groove with space enough to enter a corresponding screw thread of the screw portion 14a therein.
Second, hydraulic pressured oil is introduced into a chamber 30a for mold clamping operation formed in the die clamping cylinders 30, which generates high clamping force through the tie bars 14. After that, successive processes of injecting melt resin material into a mold through the nozzle 34 of the injection molding machine, filling and cooling are executed to produce molded articles. Then, a die opening process starts.
In the mold opening process, hydraulic pressured oil is introduced into a chamber 30b for return (mold opening in high pressure) formed in the mold clamping cylinders 30. Driving the servomotor 20 in reverse can do the mold opening operation, because the movable die plate 18 is screwed through a nut member 24 with a feed screw member 22. Furthermore, the engagement between the half nuts 28 and the tie bars 14 are released during the mold opening operation, and at the same time driving the servomotors 32 in reverse allows the tie bars 14 to move at the position “B” of FIG. 9 in the direction of left. Thus, one whole cycle of injection molding terminates.
Accordingly, in the complex mold clamping apparatus illustrated in FIGS. 6 to 9 it is possible to shorten the cycle time of one injection molding, because the half nuts 28 can open and close during the mold open and close operation.
In the above, three prior arts disclosed in Japanese laid open patents are illustrated. However, in case of initial setting, namely exchanging the mold to new one, it is necessary for any one of the prior arts to arrange the half nuts so as to be in engagement with the screw portion formed on the tie bars at the specific position, and further to memorize and hold the position mechanically or electrically.
In Japanese laid open patent number 10-296809, the engaging means moves together with the movable die plate to the tie bars, and executes engagement operation while moving. In the case, there is a disadvantage that it takes time because finally the speed of the movable die plate must be set slower, though a plurality of half nuts is provided to suppress a shock at the engagement.
In Japanese laid open patent number 10-296810, a pair of half nuts for mold clamping must be arranged on both side of the movable die plate, respectively.
Therefore, there is a disadvantage in this case that, in addition to a structural complexity another drive means is required to independently drive the half nut located at the one side of the movable die plate.
Furthermore, in the case explained above referring to FIGS. 6 to 9, it is disclosed that while the movable die plate 18 moves, the relative moving speed between the half nuts 28 and the tie bars 18 are controlled and held to become zero, and the half nuts 28 executes engagement operation under the relative moving speed “zero”. There is also a disadvantage in the case that drive control for both of the movable die plate and the tie bars are complex because the relative position between the half nuts 28 and the tie bars 18 must be adjusted at an appropriate position for engagement while both of them move.
As shown in FIG. 8, the servomotors 32 are mounted on the end portion of the mold clamping cylinders 30 to slidably move the tie bars. There is also another disadvantage in this case that two different drive means (mold clamping cylinders 30 and servomotors 32) are required for moving the tie bars at two different timings, one of which is for the mold close operation in the direction of axis of the tie bars 14 and the other is for the mold clamping operation.
The inventors of the preset invention have investigated diligently concerning such disadvantages in the prior arts, and as the result, convinced that such disadvantages could be solved by utilizing the relative speed and controlling a timing for driving the engaging means rather than controlling and holding the relative speed to be zero.